Explore high-level abstractions for tx building (vs existing low-level actions)

data points:

- provide nice ergonomics for batching commands
- command eg: define relative TTL in terms of submission date

Review the existing low-level actions and transaction-building flow first. Define how batching commands should be expressed and how a relative TTL is calculated from submission date. Done means the high-level abstractions provide clearer ergonomics for both cases without losing the existing low-level capabilities.

Written by the indexing model from the issue text.